Photolithography masks or “photomasks” are used extensively in the fabrication of integrated circuits on semiconductor wafers. Standard photomasks include a patterned light reflecting opaque layer or film on a transparent substrate. A metal, such as chromium, having a thickness on the order of about 1,000 angstroms is often used as the opaque layer or film, nickel and aluminum may also be used. Quartz is often used as the transparent substrate, though materials such as glass and sapphire can also be used. Features on the photomask can be as small as a few millionths of an inch. When the pattern is formed on the mask, typically by using computer control lasers or electron beam systems to expose the desired mask pattern in a photoresist material, it is not unusual for the mask to have defects. There are essentially two defect types, opaque and clear. Clear defects are areas where absorber is missing from areas that should be opaque; opaque defects are areas having absorber material deposited in the areas that should be clear. Since any defect in the photomask will ultimately be transferred to any semiconductor chip manufactured using that mask, these defects must be repaired before the mask can be used.
Because photomasks often have lines having a width of 0.3 mm with a width tolerance of about 10% or less, current repair techniques rely on the use of highly focus beams of photons or ions. Specifically, opaque defect repair currently involves laser evaporation or ablation or focused ion beam (FIB) sputtering of unwanted chromium in defect regions. However because the resolution of a laser is limited, if the opaque defect is connected to an adjacent chromium line, laser ablation may damage the adjacent line, removing some wanted chromium from the line. In addition, because a great deal of thermal energy is transmitted with a laser beam, the laser ablation step not only melts and vaporizes the unwanted metal defect region, but also may damage and remove a layer of the quartz underlying an adjacent opaque defect, producing roughness in the quartz. This damaged region of the quartz reduces transmission and alters the phase of the light transmitted through the mask.
As an alternative to laser ablation, FIB offers a controlled process for sputtering a small region of unwanted material. The ion beam can be focused to a much smaller size than a laser beam. In addition, the ion beam physically sputters material, transferring less thermal energy to the mask, although some damage to the quartz substrate may nevertheless occur.
FIB suffers from a number of other problems. First, because a mask is formed on a quartz substrate which is an insulating material, the ion beam rapidly changes the surface and both the ability to aim subsequent ions and to use the ion beam to image the results is degraded. Second, while an opaque defect is being removed, quartz at the edge of the defect is attacked at the same rate, and the result is a “riverbed” or trench of damaged quartz around the defect, the quartz in this region having altered transmission and phase. Third, the FIB species is typically gallium and gallium has been found implanted into the quartz when the opaque defect is removed, causing transmission losses. Finally, sputtering of material by the ion beam leads to ejection of materials in all directions and some of this ejected material comes to rest on adjacent edges.
In order to minimize some of the undesirable effects of the FIB process discussed above, a multi-step process has been used in which a rough repair is first made using FIB, following which the mask is cleaned using NaOH. Then, fine repair is made on the defect, again using an FIB process. Commercial equipment for carrying out these types of FIB repairs is commercially available from companies such as Seiko and Micrion. Following the fine repair, the mask is cleaned again with NaOH using an inductively coupled plasma, following which the defect is inspected using suitable equipment such as a microlithography simulation microscope. Nevertheless, this multi-step process often results in excessive material removal or removal of wanted material along the lines that are adjacent to the opaque defect.
Clearly, it would be desirable to provide a method for repairing opaque defects that avoids the problems associated with current techniques discussed above. The present invention is directed to solving these problems.